How can the browser tell where I am?
Totally based on registered IP address? What if I'm behind a proxy?
How can the browser tell where I am?
Totally based on registered IP address? What if I'm behind a proxy?
That would depend on the browser and its settings. It might use GPS, WLAN AP-based location, IP location, or any other conceivable method of locating itself.
The browser runs on your local machine, so even if you're behind a proxy, it should be able to find your actual IP address (unless security features prevent it from being sent for lookup to a geolocation server). Whether any browser is smart enough to handle being behind a NAT (with the local machine having a private IP) and looking up the gateway's IP address, that I don't know.
Firefox and Fennec for instance use the Google Location Service, which is also used by Google Gears.
As Matti said, it is browser-specific. Here's information on how Firefox does it: http://www.mozilla.com/en-US/firefox/geolocation/